Signalling apparatus



H. K. BRADFORD SIGNALLING APPARATUS 5 Sheets-Sheet 2 Feb. 2, 1960 Filed NOV. 14, 1955 INVENTOR HENRY K. BRADFORD FIG. 4.

j "ATTORNEY Feb. 2, 1960 H. K. BRADFoRD 2,923,882

SIGNALLING APPARATUS Filed Nov. 14, 1955 3 Sheets-Sheet 5 I \\,a l. 4 l

INVENTOR HENRY K. BRADFORD BY ATTORNEY United States Patent SIGNALLING APPARATUS Henry K. Bradford, Falls Church, Va.

Application November 14, 1955, Serial No. 546,594

12 Claims. (Cl. 324-77) This invention relates to signal transmission and reception in general. More particularly, this invention relates to signal transmission and reception over a wide portion of the spectrum by the use of electrical frequency dividers.

An object of this invention is to provide an improved frequency divider and wave analyzer.

Another object of this invention is to provide a signal receiver for signals on the order of 70 krnc. to 120 kmc. in which frequency division is accomplished without the conventional beat frequency oscillation generator.

Still another object of this invention is to provide an improved frequency divider and wave analyzer that may be employed with high frequencies as well as with audio frequencies.

A further object of this invention is to provide an improved frequency divider and wave analyzer in which the frequency division may be continuously varied over a predetermined range, which may include a very large section of the frequency spectrum limited only by the signal frequency range carried by the waveguide or other signal carrier.

Still another object of this invention is to provide an improved frequency divider and wave analyzer adapted to be employed for receiving signals in the SHF band and in which the signals are divided by a factor in the neighborhood of 1,000.

Another object of this invention is to provide an irnproved frequency divider and wave analyzer employing a signal delay channel that is provided with a dielectric in which the dielectric constant may be varied so as to control the delay of the signal passing through this channel.

Another object o-f this invention is to provide an improved delay line including a voltage sensitive dielectric for delaying the signal passing through the delay line in accordance with the magnitude of the voltage that is applied to the dielectric.

Another object of this invention is to provide an improved signal receiving apparatus in which the signal is divided into a normal signal and a delayed signal, a delayed signal being passed through a voltage sensitive dielectric in order to variably delay this signal, the normal signal and delayed signal being thereafter mixed to beat or heterodyne with one another to form a beat frequency which may be detected or demodulated and the signal components thereby received with or without amplification.

Another object of this invention is to provide a signal wave analyzer for use with signals having frequencies on the order of l kmc. -in which no beat frequency oscillator is employed.

Other and further objects of this invention will be apparent to those skilled in the art to which it relates from the following specification and claims.

In dealing with electronic techniques in many branches of use and investigation above the UHF spectrum it is found that the tunable band ranges become more limited.

Both mechanical and electrical tuning methods decrease in percentage band coverage as frequency is increased and ordinary refinement of these techniques using conventional methods has not greatly extended this coverage. The degree of this limitation is at once realized from a look at the tuning characteristics of klystrons, BWO tubes, and retarded field oscillators.

Above 10 kmc., a tuning range of 5 percent is considered exceptional when using klystrons. Retarded field oscillators have been designed for an octave coverage but these have not found their way into practice. Moreover, the compromises which must be made in the application of crystal-video techniques are so great that advantages of band width are all but lost. The diiculty in this application centers around low level mixers.

In this invention this problem was approached from an entirely new direction in a desire to produce a frequency divider and wave analyzer having the greatest possible band coverage a single simple unit.

In accordance with this invention there is provided an improved frequency divider and wave analyzer which has many applications and is particularly adapted for reception of signals in the SHF band although it may, of course, be employed in any part of the entire frequency spectrum. Briey, this system consists of a circuit which splits the signal wave or input energy into two components. This circuit may consist of a wave guide, a coaxial line, a paired line or other signal channel. One of the split signals is passed through a normal circuit or channel whereas the other of the split signals is passed through a circuit or channel that is fitted with a voltage sensitive dielectric in which the dielectric constant will vary with the application of a D.C. voltage to the electrodes thereof. Thus, the signal passing through the dielectric can be delayed in predetermined amounts depending upon the voltage applied to the dielectric.

The signal component passing through the normal channel and the signal component passing through the delay channel are brought together at the outputs of these channels and are mixed to form a beat frequency or heterodyne signal. At the same time the signal may be detected or if the signal is small in amplitude an auxiliary oscillator may be used so that the crystal devices used for detection will function properly. The heterodyne signal may then be amplified by suitable amplifiers. Where this invention is employed With very high frequencies, such as those encountered in the SHF band, a number of amplifiers adjusted for amplifying adjacent and overlapping bands may be employed so that the entire band width of the heterodyne signals will be properly amplified. These and other details of this invention will be set forth in the following specification and the drawing, in which briefly:

Fig. l is a schematic drawing of an embodiment of this invention;

Fig. 2 is a modification of this invention;

Fig. 3 is a detail sectional view of one form of progress signal delay channel adapted to be employed in accordance with this invention;

Fig. 4 is a schematic drawing of the apparatus shown in Fig. 1 coupled to a group of amplifiers;

Figs. 5 and 6 are views showing modified forms of mixers adapted to be employed in accordance with this invention;

Fig. 7 is a view showing signal waveform and average dielectric constant graph used to explain the operation of the mixers shown in Figs. 5 and 6;

Fig. 8 is a view of a visual indicator adapted to be employed in accordance with this invention;

Fig. 9 is a View showing a modified form of this invention adapted for use with lower frequency signals of audio or super audio ranges or signals up to ten megacycles;

Fig. 10 is a view of a modification showing two units of the type shown in Fig. 1 connected in cascade; and

Fig. 11 is a view of a modified form of this invention applied to open sided signal carriers such as are used in signal measurements.

Referring to the drawing in detail, there is shown in Fig. l an embodiment of this invention in which a signal channel 10, such as a waveguide, is divided into two sim.- ilar channels 11 and 12 so that equal components of the signal are introduced into each. Channel 11 may be referred to as the normal channel and channel 12 as the progressive delay channel.

Similar dielectrics 13 and 14 are placed into the channels 11 and 12, respectively, and these dielectrics are selected to introduce considerable delay into the signal components with a minimum of loss. The dielectric 14 must have a voltage sensitive characteristic such that its dielectric constant may be varied as a function of voltage applied thereto by a satisfactory amount. A suitable dielectric for this purpose is barium strontium titanate.

A saw-tooth voltage or other voltage source 15 is connected to the electrodes 16 and 17 that are applied to the dielectric 14. These electrodes are insulated from the waveguide 12 so that the voltage supplied by the source 15 is not short circuited through the waveguide. Other ways of accomplishing this will be described hereinafter.

The rate of voltage change of the source 15, that is, its differential or rate in volts per second is made to match the voltage dielectric function of the voltage sensitive dielectric 14. This is done to produce a signal component E having a linear frequency shift, emerging from the end 14a of the dielectric 14. This frequency shift will differ from that of the signal component D emerging at the end 13a of the dielectric 13 by a fixed amount for the duration of the changing voltage function.

If the dielectric 14 is one having a dielectric constant of 100 with a loss factor of .0003, it has a voltage sensitivity of -60 percent per 500 volts. Let us assume that for a change of dielectric constant of 100 percent, 200 volts is required. As the propagation velocity of the signal through the dielectric 14 is proportional to the square root of the dielectric constant, it follows that the velocity of propagation will change approximately percent with a 200 volt change.

For the purpose of describing the operation of this invention, assume that a kmc. signal is progressing through the waveguide at the rate of 10,000 wavefronts per microsecond. A substance 14 placed in the waveguide having a delay of V10 microsecond will therefor contain 1,000 wavefronts. If this substance has a dielectric constant K of 100 it would have to be 10 freespace wavelengths long to contain 1,000 wavefronts at one time.

If the velocity of propagation is decreased 5 percent per microsecond, the dielectric must contain 1050 wavefronts. It accumulated 50 additional wavefronts each microsecond and the signal frequency is thus reduced 50 mc., over a one microsecond interval. However, the dielectric is acting on the signal only during 1/10 microsecond in which case the frequency difference is only 5 mc. This requires the voltage slope to be 200 volts per microsecond which is a value that may be diiicult to attain. At a slope of 200 volts per millisecond the frequency difference will be 5 kc., which is not very useful in this signal frequency range. However, the loss involved is negligible.

In order to overcome the above difficulties, a dielectric 14 with a constant K equal to 1000 was selected. By making it occupy a free space wavelength of 10 wavelengths, it will contain 10,000 wavefronts and will produce a delay of one microsecond. Acting on l0 times the field length and having 10 times the voltage sensitivity, the frequency reduction factor in the dlelectric 14 will be incresed 100 fold. A beat frequency of 0.5 mc., will thus be formed. Further by using a dielectric having a constant K=6,000, we multiply the factor by 36 or the square of 6 because the voltage sensitivity and the physical volume of material acted upon are both increased by a factor of 6. This brings the beat frequency up to 18 mc. At 18 mc., we have achieved a division ratio of 10 kmc./ 18 mc.=557.

The voltage from the source 15 may be impressed across both of the dielectrics 13 and 14 but in opposite phase so that the rate of signal delay in one dielectric is increasing while the rate in the other is decreasing. Such an arrangement is shown in Fig. 2 in which the dielectric 13 is provided with electrodes 19 and 20 that are connected to the voltage source 15. The beat frequency will in this modification be doubled to 36 mc., from the 18 mc. value obtained when the voltage is applied only to the dielectric 14.

The frequency division ratio is controlled directly by the slope of the saw-tooth or dielectric function voltage. This voltage-dielectric function may be made linear or non-linear as desired. The signal components emerging from the dielectrics 13 and 14 are recombined in the waveguide 18 to which the branches 11 and 12 are joined and thereafter the combined signal components are simultaneously mixed by a conventional crystal 21 or by a new type of mixer to be described.

The dielectric 14 may be mounted in a waveguide section 12a as shown in Fig. 3. The input end 12b of this section is coupled through the gap type coupler 12e to the waveguide 12 which is one of the branches shown in Fig. 1. The output of the section 12a is coupled through a similar gap type coupler 12d to provide the signal E.

In the section 12a there is provided a gap 12e which extends from the input end to the output end both on the top and the bottom of the section. This gap divides the section into two parts and these parts are connected across the output of the saw-tooth oscillator 15 so that the saw-tooth voltage is applied across the dielectric from one side to the other. It is, of course, obvious that a gap 12e may be provided to each of the sides of the section 12a instead of the top and bottom as shown, if it is desired to apply the saw-tooth across the dielectric from the top to the bottom and vice versa.

In order for the crystal 21 to operate in the most favorable portion of its operating characteristic for high mixing efficiency, an auxiliary oscillator 23 may be used to drive the crystal. The frequency of the auxiliary oscillator is chosen well out of the band-pass of the LF. or sorting amplifier 22 and the emerging signal D plus or minus E is amplified to the extent that may be required. The relation D plus or minus E bears a fixed ratio to the input signal A, which ratio is determined by the slope of the saw-tooth voltage function as previously described.

The oscillator 23 may employ a klystron type tube in a Shepherd-Pierce circuit and the I F. output from this beat frequency oscillator may be connected to the inputs of a series of LF. amplifiers 23a, 23b, 23e, 23d and 23e, each covering a different frequency range, as shown in Fig. 4. Amplifier 23a is adjusted to amplify a band from 3 to 6 mc., amplifier 23b a band 6 to 12 mc., amplifier 23o a band 12 to 24 mc., amplifier 23d a band 24 to 48 mc., and amplifier 23e a band 48 to 96 rnc., whereby the whole LF frequency range 3 to 96 mc., of the signal derived from the beat frequency oscillator may be amplified.

In Figs. 5 and 6 there are illustrated low level types of mixers that employ barium titanate elements 21a instead of a crystal detector such as the detector 21 shown in Fig. 1. One side of the element 21a is cemented to a wall of the waveguide 18a and a connection 2lb is cemented to the other side of the dielectric element 21a.

The member 2lb may be in the form of an inductive pick-up loop as shown in Fig. 5 or it may be in the form of a post probe as shown in Fig. 6. In this way the signal current from both the D and E signals is produced in the device shown in Fig. 1 and supplied to the mixers shown in Figs. 5 and 6 and these are impressed upon the dielectric material 21a in the form of a complex wave such as is illustrated in Fig. 7. This complex wave appears as a high frequency wave modulated in amplitude by the beat frequency derived from the signals D and E. This beat frequency may, of course, be the difference between the frequencies of the signals D and E. The amplitude of the beat frequency will change the average dielectric constant of the dielectric element 21a as schematically represented in the lower curve shown in Fig. 7. Hence the capacitance of the element 21a is changed.

A small D.C. voltage is supplied from the source 21e and this voltage is impressed across the dielectric element 21a through a suitable choke coil 21d which is shunted by capacitor 21e. The impedance of this tuned circuit is as high as possible at the inter-modulation frequency.

The capacitor, including the dielectric 21a, will change its average voltage at the inter-modulation frequency in an attempt to charge and discharge as a consequence of its changing capacitance. This capacitance includes the capacitance of the low pass network including the capacitors 21g and 21h. This network may be connected to one or more sorting amplifiers such as the amplifiers 23a-23e shown in Fig. 4. Thus, the voltage change at inter-modulation frequencies is amplified and supplied to other signal circuits as may be desired and one such circuit comprises a visual indicator.

A visual indicator circuit such as may be employed in connection with the sorting amplifying circuit shown in Fig. 4, is shown in Fig. 8. This circuit is provided with a cathode ray tube 50 having a conventional electron gun which is arranged to produce a flat fan-shaped electron beam. This tube is provided with a fiat deflector electrode 51 that is connected to one side of the outputs of all of the amplifiers 23a-23e- The fiat fanshaped beam is projected below the fiat electrode 51 and above the narrow strip electrodes 52 that are positioned below the fiat electrode 51 and spaced therefrom. With no signal applied to the electrodes 51 and 52 from the outputs of the amplifiers, the fiat fan-shaped beam impinging upon the fluorescent screen S3 of the tube 50 will produce aV horizontal line along the zero ordinate of the scale applied to the front of the screen of the tube. The abscissa of this chart may be calibrated in terms of frequency corresponding to the frequencies covered by the amplifiers 23a-23e. The amplifiers 23a- 23e in this case may be wide band logarithmic video amplifiers connected so that all amplitude modulated signals provide a deflection upward from the zero line as shown by the indications A, B, D and E. Discriminator outputs resulting from a frequency modulated signal are polarized to defiect a beam downward as shown by the indication C.

The output of the sorting amplifier is fitted with a number of band pass filters overlapping to form a continuous spectrum but each filter has its own detector, amplifier (video) and defiection plate. A wide band logarithmic video amplifier allows an upward defiection of all A.M. signals such as the 800 pv. signal shown at A (4 krnc., modulated C.W.), the 60 nv., 8 krnc. signal shown at B, C.W. pv. signal shown at C, and the 7 nv., 60 kmc. pulse shown at E. Discriminator outputs are polarized to defiect downward such as shown at C to distinguish them from A.M. signals. Here an 8 /.1.v., 12 kmc. F.M. pulse is indicated.

Frequency is arranged to be dispersed logarithmically on the horizontal of the screen by the filter design, while magnitude is indicated vertically on a logarithmical response scale. Pulses will form a vertical line while C.W. will form an elevated point, the height of which is a measure of signal magnitude.

In Fig. 9 there is shown a modified form of this invention in which voltage sensitive capacitors 25, 26, 27, 28, 29 and 30 are connected into a lumped delay line 24 that also includes the inductance coils 31, 32, 33, 34, 35 and 36. The voltage sensitive capacitors 25, 26, 27, 28, 29 and 30 are provided with dielectrics 37, 38, 39, 40, 41 and 42 respectively, and these dielectrics are of the barium strontium titanate type having characteristics similar to the dielectric 14 shown in Fig. 1 and described in connection therewith.

The inductance coils 31 to 36 inclusive, are of the type having current sensitive permeability and for this purpose these coils may be provided with ferromagnetic ferrite cores having the desired electromagnetic proper ties.

This modification of this invention is designed for use with lower frequency signals than the embodiment shown in Fig. 1 and these lower frequency signals extending over a wide frequency band are applied to the input lines 43 and 44. Part of the signal is fed to the variable delay channel including inductance coils 31-36 and capacitors 25-30 and the other part of the signal is fed to the fixed delay line 45. The oscillation generator 46 is of the type adapted to generate a voltage having saw-tooth waveform and this generator is connected across the delay line capacitors 25-30 so as to vary the voltage sensitive dielectric thereof as previously described in connection with Fig. 1 Suitable chokes or couplers are employed for preventing the signal from going into the generator 46.

It is obvious that the delay line 45 may be constructed the same as the variable delay line 24. In that case both delay lines 24 and 45 are variable, one increasing signal delay while simultaneously the other is decreasing, and in that modification the generator 46 is also connected to the delay line 45 for the purpose of applying a varying voltage to the voltage sensitive dielectric elements thereof.

Several of the units such as shown in Fig. 1 may be employed in cascade as illustrated in Fig. 10 wherein the output of the mixer 21 of the first unit 11 is coupled through the coaxial line 18a to the input of the second stage 11a. Tnis second stage is similar to the first stage in that it also provides two signal paths, one of which may be referred to as a normal signal channel and the other of which may be referred to as a channel with periodically progressively delayed signal characteristics. It is, of course, obvious that both channels of stages 11 and 11a shown in Fig. 10 may be of the type shown in Fig. 2 wherein both signal paths in each stage have periodically progressively delayed signal characteristics. The result of this modification is a greatly increased frequency division in the mixer circuit coupled to the output 18b of the second stage 11a. It is, of course, obvious that the output of the stage 11a may be coupled to the input of a stage such as is shown in Fig. 9 if desired.

A still further modified application of this invention is shown in Fig. 11 wherein sections of dielectric 60 and 61 are positioned in the open side type of SHF wave carrier 62 that is adapted for use in certain measurements. A conductor 63 is positioned in this carrier and access may be had to different points of this conductor through the open side of the carrier for the purpose of making measurements. Suitable electrodes 64 and 65 are provided to the dielectric elements 60 and 61 and these electrodes extend along the full length of the dielectric elements. These electrodes are insulated from the walls 60 and 61 of the carrier and they are also connected by conductors 66 and 67 respectively, to the saw-tooth oscillator 69, the center conductor 68 of which is connected to the conductor 63 of the carrier so that voltages of the saw-tooth waveform shown are applied between these electrodes 64 and 65 with respect to the conductor 63.

As a result the dielectric elements 60 and 61 both function to introduce progress signal delay into the carrier as the amplitude ofthe saw-tooth voltage increases.

This invention is susceptible of various modifications and applications in the communications lfield and it is therefore not desired to limit it to the embodiments described in the foregoing specification except insofar as they are defined by the following claims.

What I claim is as follows:

1. A wide band electrical frequency divider comprising a radio frequency signal channel, means for dividing said channel into a normal channel and a signal delay channel, said delay channel having electrically responsive means for introducing a time rate of change in the Vdelay of transmission ofthe signal therethrough, means for mixing the output signals from said normal channel and from said delay channel to produce a signal of reduced Afrequency, the reduction in frequency being-determined by the time rate of change of delay in said signal delay channel.

2. A wide band electrical frequency divider comprising I a radio frequency signal channel, said channel being divided into a normal channel and a signal delay channel, said delay channel having electrically responsive means for progressively increasing the delay of transmission of the signal therethrough, means for mixing the output signals from said normal channel and from said delay channel to produce a signal of reduced frequency, the reduction in frequency being determined by the time rate of change of delay in said signal delay channel.

3. Signalling apparatus comprising a radio frequency signal channel, means for dividing said channel into a normal channel and a signal delay channel, a voltagey sensitive dielectric in said delay channel yfor delaying the transmission of the signal therethrough, a source of variable voltage connected to said dielectric for introducing a time rate of change in the signal delay in said dielectric n accordance with said voltage, means for mix-r ing the output signals from said normal channel and from said delay channel to produce a signal of reduced frequency, the reduction in frequency being determinedl by the time rate of change of delay in said signal delayy channel.

4. Signalling apparatus as set forth in claim 3 further characterized in that said voltage sensitive dielectric comprises a dielectric of barium strontium titanate.

5. Signalling apparatus as set forth in claim 4 further.

characterized in that the source of variable voltage generates a sawtooth voltage wave.

6. Signalling apparatus as set forth in claim 5 further characterized in that the normal channel is` also provided with a voltage sensitive dielectric, said last mentioned di, electric being connected to said voltage source so that the dielectric constant thereof varies inversely as the dielectric constant of said dielectric in said delay channel.

7. Signalling apparatus as set forth in claim 3 in which the delay channel comprises a series of lumped inductors and capacitors and in which the voltage sensitive dielectric forms the dielectric for said capacitors.

8. Signalling apparatus as set forth in claim 3 further comprising a plurality of amplifiers connected to said signal mixing means, each of said amplifiers being adapted to amplify a different frequency spectrum covered by the signalling apparatus and a visual indicator comprising a cathode ray tube having a plurality of beam deliecting means, different ones of said beam detlecting means being connected to the output of different ones of said ampliers.

9. Signalling apparatus as set forth in claim 3 further characterized in that said signal mixing means comprises a voltage sensitive dielectric which is connected so that the dielectric constant thereof varies in accordance with the voltage of the output signal.

10. Signalling'apparatus as set forth in claim 3 further characterized in that one of said channels is provided witha'conductor positioned inside thereof and in which the voltage sensitive dielectric is positioned on the sides 0f said conductor. l `11.- Signalling apparatus as set forth in claim 10 further characterizejdin that said source of voltage is connected between said conductor and the channel associated therewithp.' f

Y 12. A wideband electrical frequency divider comprising a radio frequency signal channel, means for dividing said channel into a normal channel and a signal delay channel, a voltage sensitive dielectric in said delay channel for delaying the transmission of the signal therethrough, said dielectric having a length such that it contains several thousand signal wave fronts at a given instant, means for periodically varying the dielectric constant of said dielectric so that a progressively increasing delay in signal transmission is introduced therein from thetime-a signal wave front enters said dielectric until it leaves, and means for mixing the output signal from said normal ychannel with the output signal from said delay channel to produce a signal of reduced frequency, the reduction in frequency being determined by the time rate of change-of delay in said signal delay channel.

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